Adsorption of CH4on the Pt(111) surface: Random phase approximation compared to density functional theory

Abstract

We investigate the adsorption of CH4 on the Pt(111) surface for two adsorption modes, hcp (hexagonal closed packed) hollow tripod and top monopod in a (√3 × √3)R30° surface cell that corresponds to experimental surface coverage. Surface structures are optimized with density functional theory using the Perdew-Burke-Ernzerhof (PBE) functional augmented with the many-body dispersion scheme of Tkatchenko (PBE+MBD). Whereas the Random Phase Approximation (RPA) predicts a clear preference of about 5 kJ mol-1 for the hcp tripod compared to the top monopod structure, in agreement with vibrational spectra, PBE+MBD predicts about equal stability for the two adsorption structures. For the hcp tripod, RPA yields an adsorption energy of -14.5 kJ mol-1, which is converged to within 1.0 ± 0.5 kJ mol-1 with respect to the plane wave energy cutoff (500 eV), the k-point mesh (4 × 4 × 1), the vacuum layer (about 10.3 Å, with extrapolation to infinite distance), and the number of Pt layers (3). Increments for increasing the number of Pt layers to 4 (+1.6 kJ mol-1) and the k-point mesh to 6 × 6 × 1 (-0.6 kJ mol-1) yield a final estimate of -13.5 ± 2.1 kJ mol-1, which agrees to within 2.2 ± 2.1 kJ mol-1 with experiment (-15.7 ± 1.6), well within the chemical accuracy range.